Time delay chemical fuse



y 1965 R. c. PUYDAK 3,196,235

TIME DELAY CHEMICAL FUSE Filed Oct. 51. 1961 INV EN TOR. E0657"; 6.

r flaw m United States Patent M 3,196,235 TIME DELAY (ZHEMECAL FUSE Robert C. Puydak, West Grange, Ni, assignor to McGraw-Edison Company, Milwaukee, Win, a corpo= ration of Delaware Filed Get. 31, 1961, Ser. No. 143,973 v '7 Qiaims. (l. 206-435) This invention relates generally to circuit interrupting devices and more particularly to electrical fuses of the type whici are adapted to carry relatively large overloads for a short period of time or relatively small overloads for a longer period of time.

Thermal time delay fuses and other types of lag fuses are well known and are frequently utilized to protect electric motors or other electrical equipment which draw large current surges incident to starting. These fuses must of necessity be capable of accommodating quick initial surges without blowing.

Prior art lag fuses have tended to provide the necessary time delay by means of electrical or mechanical cutout mechanisms which are relatively expensive and which increase the size of the fuse. In addition such fuses tend to be quite fragile due to the delicate fuserelement positioned therewithin.

I have found that a particularly designed chemical fuse which utilizes the reaction between certain of its components to interrupt an electrical circuit offers a means of avoiding the limitations inherent in prior art fuses.

In accordance with this it is an object of this invention to provide a simple inexpensive time delay fuse.

Another object of this invention is to provide a small compact electrical fuse.

Still another object of this invention is to provide a heat actuated circuit interrupting device that may be pot-ted or packed in such a manner that vibration resistance is maximized.

A further object of this invention is to provide a fuse which may be constructed in a variety of shapes and sizes.

Another object of this invention is to provide a chemical fuse which is ordinarily conducting but which develops a resistance sufficient to interrupt a circuit upon predetermined overcurrents therethru.

A still further object of this invention consistent with the foregoing objects is to provide a simple method of making a reliable, accurate circuit interrupting device.

Other objects and advantages of this invention will be apparent from the following description of the preferred embodiments of the invention taken in connection with the accompanying drawings in which:

FIG. 1 is a cut away perspective view of the fuse.

FIG. 2 is a cut away perspective View of the fuse of FIG. 1 after the operation thereof.

FIG. 3 is a plan view of the contact used with the fuse of FIG. 1.

FIG. 4 is a view in elevation of the contact of FIG. 3.

*IG. 5 is a plan view of a modified contact adapted for use with the fuse of FIG. 1.

FIG. 6 is a plan view of another modified contact.

FIG. 7 is a plan view of another modified contact.

PEG. 8 is a plan view of still another somewhat modihad contact which like the other modifications is particularly suited for use in conjunction with the fuse herein described.

Referring now to FIG. 1, It) indicates generally the pellet type fuse of this invention. The fuse comprises a body portion 11 having incoming and outgoing leads 1?. and 13 respectively, afiixed thereto. The leads may be soldered or otherwise secured to contact members 14 (FIGS. 3 and 4). The contacts comprise a conductive screening which is in intimate contact with the body por- .tion ll of the fuse. The main prerequisites of the corn 3,196,235. Patented July 20, 1965 as in tact member 14 and those contacts to be discussed subsequently are that they be conducting and of such a form that the body material can flow into or around them to create a good electrical contact and a firm bond therebetween.

FIGURES 5-8 indicate various modifications of the contact member. The member 15 of FIG. 5 comprises a thin disk 16, of for example, copper sheet which is provided with a lead or similar coating 17. The lead coating in this modification serves to form a bond between the conductive copper and the body of the fuse. While a lead coated copper sheet is illustrated it is entirely possible to replace the lead with another substance which will flow under pressure and to replace the copper with another conductive material.

FIG. 6 shows a conductive disc 18 which is provided with a plurality of apertures 19 therein. The disc may be of copper, nickel, silver or any like conducting material. The perforations allow the fuse body material to enter the disc when pressure is applied to the contacts to thereby achieve a good electrical and mechanical bond between the disc and the body.

FIG. 7 illustrates a cross-shaped complex of conducting wires that may serve as a contact 2%. While it is not shown single crossed wire pairs could also be used to advantage. Here again when pressure is applied to the contacts which are positioned on either side of the fuse body, the body material will electrically and mechanically bond to the wires.

The embodiment of FIG. 8 is a spiral shaped conductive wire that may serve as a contact 21. As before the shape of the contact aids in providing good electrical contact between the leads and the fuse body as well as providing a structure which is easily afiixed to the fuse body.

While it is not shown it should be understood that lead wires are mechanically and electrically ailixcd to each of the contacts illustrated in FIGS. 5-8. The contacts may be made of copper sheet, perforated nickel-plated steel sheet, crossed nickel wires, platinum screening or any similar material. The contacts may also take forms different from the ones discussed without deviating from the scope of this invention. The body of the fuse may also take any number of sizes and shapes depending upon the preference of the user and the duty to which the fuse is to be subjected.

A satisfactory form of the subject invention may be constructed by soldering or otherwise afiixing lead Wires to each of two contact plates. The plates may then be placed in a cylindrical die with a metal-salt mixture juxtaposed therebetween. Pressure is then applied to the contacts so as to compress the mixture and thereby form a cohesive cylindrical pellet with wires protruding from opposite sides. When the pellet is inserted in a circuit current will normally flow thru one lead wire across the compressed mixture and thru the other lead wire. However on overcurrents, electrical heating will occur which causes a chemical reaction between the metal and the salt. This reaction produces a reactive product which serves to increase the resistivity of the pellet so as to effectively terminate the current flow therethru and thereby interrupt the electrical circuit.

Various metal-salts may be utilized to good advantage lead to 1.9 grams of bismuth iodide. .25 gram of this mixture was compressed between copper wire mesh contact plates composed of .01 diameter wire, 40 wires per inch, at approximately 85,500 p.s.i. to form a pellet approximately A in diameter and thick with a range of internal resistances between .005 and .040 ohm. Fuses produced in the above manner having an internal initial resistance of approximately .020 ohm or aged to this resistance at 105 C., will have a time characteristic similar to that of a 3 or 4 amp. conventional slow burning fuse. The above described fuse on a 6 volt circuit, for example, will develop after reaction resistances up to 7 megohms.

Another fuse was prepared by using 200 mesh tin powder in the ratio of 1 gram of tin for every 3.3 grams of bismuth tri-iodide. When fabricated as above these fuses had an internal resistance of approximately .04 ohm. Upon insertion in a 3 volt A.C. circuit with a current of 3 amps, the fuses interrupted the circuit in approximately 1 /2 minutes by increasing their resistance to 250 ohms.

Other fuses have been prepared by using eutectic alloys. One form of this type fuse was prepared by utilizing a lead-tin binary eutectic alloy (62% lead, 38% tin) in the form of filings less than 100 mesh in size. For each gram of alloy used 2.43 grams of bismuth tri-iodide were used.' As above the mixture was compressed between contact plates of lead coated copper sheet into a thin cohesive wafer; the soft lead coating forming a bond between the copper and the reactive mixture. Upon insertion in a 6 volt A.C. circuit with a current of 6 amperes the fuse interrupted the circuit in approximately 20 seconds by increasing its resistance from less than .1 ohm to several thousand ohms.

I have found that cohesive pellets may be formed by subjecting the various mixtures to pressures between 7,000 and 85,000 psi with the pressures near the upper range being somewhat preferred.

It should be noted that fuse elements constructed in the manner described will function properly, immediately after fabrication. However changes in the internal resistance of the fuses are observed to occur with time. This change of resistance is relatively slow at room temperatures but at 105 C., for example, the change occurs relatively rapidly. At an intermediate temperature such as 65 C. the resistance of a lead-bismuth iodide fuse increases between 3 and 4 times in l-200 hours, remaining relatively constant thereafter. This observed phenomena may be utilized to increase the uniformity and stability of the fuses by fabricating the fuses with a lower resistance and subsequently aging them to the desired resistance. With this in mind I have prepared fuses having initial resistances of .005.015 ohm and subsequently aged them for 2040 hours at 105 C. to achieve resistances of .020 ohm.

Fuses of the described type have been fabricated with relative ease in an economic manner and have been found to be accurate and reliable in use. Such fuses may also be made extremely small and in a wide variety of shapes and sizes. A great deal of vibration resistance may also be imparted to the device by either partially or completely potting or packing the fuse in a body of mineral powder.

While the described fuses are particularly efficient when utilized as current responsive elements they may also be used with great success as overheat safety devices.

That is, the surrounding temperature initiates the reaction rather than the heat accompanying relatively high current flow. Fuse devices made with tin, tin-lead eutectic or lead as the metal have operated successfully at temperatures from 270 to 400 C.

While a number of embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that various changes and modifications can be made therefrom without departing from the invention and, therefore, it is intended for the appended claims to cover all such changes and modifications as fall within the true spiritand scope of the invention.

1 claim:

1. A time delay fuse for interrupting an electric circuit in the event of the flow of excessive current in said circuit, said fuse comprising a pellet normally exhibiting low resistance to the flow of electric current and conductive means for connecting said pellet in series relationship in said circuit so that it is heated by the flow of current in said circuit, said pellet including a compacted mixture of fine mesh particles of bismuth iodide and fine mesh particles of a metal selected from a group consisting of lead, tin, and alloys of lead and tin, said mixture when heated to a predetermined temperature being con verted by the reaction between said bismuth iodide and metal particles into a material exhibiting high resistance to the flow of electric current.

2. A normally electrically conductive time delay device for interrupting an electric circuit in the event of the flow of excessive current in said circuit, said device including a body portion and conductive means for connecting said body portion in series in said electric circuit so that it is heated by the flow ofcurrent in said circuit, said body portion including a compacted mixture of fine mesh particles of bismuth iodide and fine mesh particles of a metal selected from the group consisting of tin, lead, and alloys of tin and lead, said mixture normally exhibiting low resistance to the flow of electric current and when heated to a predetermined temperature being converted to a high resistance material.

3. A time delay device for interrupting an electric circuit in the event of the how of excessive current in said circuit, comprising a normally electrically conductive pellet and conductive means for connecting said pellet in series in said circuit so that it is heated by the flow of current in said circuit, said pellet including a compacted mixture of fine mesh particles of bismuth iodide and fine mesh particles of a metal adapted to react with said bismuth iodide when heated to a predetermined temperature to form a high resistance material.

4. A time delay fuse for interrupting an electric circuit in the event of the flow of excessive current in said circuit, said fuse comprising a pellet normally exhibiting low resistance to the flow of electric current and conductive means for connecting said pellet in series in said circuit so that it is heated by the fiow of current in said circuit, said pellet including a compacted mixture of fine mesh particles of bismuth iodide and fine mesh particles of a metal selected from the group consisting of lead, tin, and alloys of lead and tin, said mixture when heated to a predetermined temperature being converted to a material exhibiting high resistance to the fiow of electric current, i

said conductive means including metallic contact members adjacent the ends of said pellet having openings therethrough permitting said fine mesh particles to pass therethrough prior to and during compaction and said contact members being intimately bonded mechanically to and in electric contact with said compacted mixture.

5. A time delay fuse for interrupting an electric circuit in the event of the flow of excessive current in said circuit, said fuse comprising a pellet normally exhibiting low resistance to the flow of electric current and conductive means for connecting said pellet in series in said circuit so that it is heated by the flow of current in said circuit, said pellet including a compacted mixture of fine mesh particles of bismuth iodide and fine mesh particles of a metal adapted when heated to a predetermined temperature to react with said bismuth iodide to form a high resistance material, said conductive means including metallic contact members at the ends of said pellet intimately bonded mechanically to and in electric contact with and spaced apart by said compacted mixture.

6. A time delay fuse for interrupting an electric circuit in the event of the fiow of excessive current in said circuit,

said fuse comprising a generally cylindrical pellet normally exhibiting low resistance to the flow of electric current and conductive means including a pair of spaced apart metallic contact members adjacent the ends of 'said pellet for connecting said pellet in series relation in said electric circuit so that it is heated by the flow of electric current in said circuit, said contact members having apertured surfaces perpendicular to the axis of said cylindrical pellet and said pellet including a compressed mixture of particles of sufiiciently fine mesh to pass through the apertures in said contact members prior to and during compression and said particles being of bismuth iodide and of a metal adapted to react with said bismuth iodide when heated to a predetermined temperature to form a product having high resistance to the flow of electric current, said contact members being surrounded by and in intimate electrical contact with said mixture.

7. A time delay device in accordance with claim 1 wherein the proportion of said bismuth iodide to said metal is approximately in accordance with the stoichiometric ratio.

References Cited by the Examiner UNITED STATES PATENTS BERNARD A. GILHEANY, Primary Examiner. 

1. A TIME DELAY FUSE FOR INTERRUPTING AN ELECTRIC CIRCUIT IN THE EVENT OF THE FLOW OF EXCESSIVE CURRENT IN SAID CIRCUIT, SAID FUSE COMPRISING A PELLET NORMALLY EXHIBITING LOW RESISTANCE TO THE FLOW OF ELECTRIC CURRENT AND CONDUCTIVE MEANS FOR CONNECTING SAID PELLET IN SERIES RELATIONSHIP IN SAID CIRCUIT SO THAT IT IS HEATED BY THE FLOW OF CURRENT IN SAID CIRCUIT, SAID PELLET INCLUDING A COMPACTED MIXTURE OF FINE MESH PARTICLES OF BISMUTH IODIDE AND FINE MESH PARTICLES OF A METAL SELECTED FROM A GROUP CONSISTING OF LEAD, TIN, AND ALLOYS OF LEAD AND TIN, SAID MIXTURE WHEN HEATED TO A PREDETERMINED TEMPERATURE BEING CONVERTED BY THE REACTION BETWEEN SAID BISMUTH IODIDE AND METAL PARTICLES INTO A MATERIAL EXHIBITING HIGH RESISTANCE TO THE FLOW OF ELECTRIC CURRENT. 